1. Field of the Invention
The present invention relates to a method of controlling the speed reduction ratio of a continuously variable transmission for use on a motor vehicle, and more particularly to a method of controlling the speed reduction ratio of such a continuously variable transmission in response to the driver's operation for acceleration or deceleration, e.g., the depression of an accelerator pedal, the shifting of a manual shift lever, or the like.
2. Description of the Prior Art
According to one conventional method of controlling the speed reduction ratio of a continuously variable transmission on a motor vehicle, a target engine rotational speed Neo is set depending on, for example, the accelerator opening which may be the depth to which the accelerator pedal is depressed, or the opening of an engine throttle valve, and then an actual engine rotational speed Nea is controlled into conformity with the target engine rotational speed Neo that has been set. However, the conventional control method tends to suffer a problem in the control of the speed reduction ratio when the engine rotational speed is in a transient state.
Such a problem will be described by way of example in detail below. It is assumed, as shown in FIG. 20 of the accompanying drawings, that while the motor vehicle is cruising at a vehicle speed V1 with an engine rotational speed N1 as indicated by a point A in FIG. 20 (at this time, a target engine rotational speed corresponding to the accelerator opening is the same as the actual engine rotational speed N1), the accelerator pedal is depressed to increase the target engine rotational speed up to N2.
To increase the target engine rotational speed, the continuously variable transmission may be controlled to bring the speed reduction ratio into "LOW" position, i.e., to increase the speed reduction ratio. As the accelerator pedal is depressed, the output power of the engine is increased, resulting in an increase in the reserve horsepower of the engine. Eventually, the relationship between the engine rotational speed and the vehicle speed shifts from the point A to a point B where the engine rotational speed is N2 and the vehicle speed is V2.
The reserve horsepower of the engine means the engine horsepower as it increases when the accelerator pedal is depressed, and serves as a horsepower to accelerate the motor vehicle. While the motor vehicle is cruising, for example, the engine output power and the horsepower required to move the motor vehicle balance each other, with the reserve horsepower being nil. When the accelerator pedal is then depressed, the engine horsepower is increased, and such an increase in the engine horsepower is called a reserve horsepower. The reserve horsepower is used as the horsepower to accelerate the motor vehicle.
If the speed reduction ratio of the continuously variable transmission is controlled using only the target engine rotational speed Neo as a value to be controlled, then when the accelerator pedal is depressed, the continuously variable transmission is controlled to change the speed reduction ratio toward "LOW" position in order to bring the engine rotational speed into conformity with the target value N2. At this time, since the engine rotational speed is increased due to the reserve horsepower of the engine, the continuously variable transmission is then controlled to change the speed reduction ratio toward "TOP" position in order to accelerate the motor vehicle with the reserve horsepower of the engine. During the above control process, however, the engine rotational speed tends to overshoot, or the speed reduction ratio changes in a manner to make the driver feel strange.
In view of the above drawbacks, the applicant has devised a control method as disclosed in Japanese laid-open patent publications Nos. 63-53343, 63-53344, 63-53345, 63-53346, 1-93662, 1-120475, and U.S. Pat. No. 5,166,877, for example.
According to the disclosed control method, the rate of change di/dt of the speed reduction ratio of a continuously variable transmission (i: speed reduction ratio) is calculated as the sum of a component (referred to as "power term di(P)") corresponding to a predicted acceleration obtained from the reserve horsepower of the engine and a component (referred to as "engine rotation term di(Ne)") corresponding to a target rate of change for the engine rotational speed. The continuously variable transmission is controlled using the rate of change di/dt of the speed reduction ratio as a value to be controlled.
Specifically, when the accelerator pedal is depressed while the motor vehicle is cruising at the point A in FIG. 20, a vector C3 is calculated by adding a vector component C1 of the rate of change of the speed reduction ratio, which corresponds to a predicted acceleration obtained from the reserve horsepower of the engine, and a vector component C2 of the rate of change of the speed reduction ratio, which corresponds to a target rate of change for the engine rotational speed. Then, the continuously variable transmission is controlled to change the speed reduction ratio from i1 to i2 depending on the calculated vector C3.
The above control method has no problem when the motor vehicle is cruising or is slowly accelerated or slowly decelerated. However, when the motor vehicle runs in a transient state, i.e., when the accelerator is abruptly depressed for accelerating the motor vehicle, i.e., for largely varying the speed reduction ratio, the value to be controlled changes toward the target value in a manner that does not satisfy the request from the driver. As a result, the driver does not feel good in such a transient running state. The process of controlling the continuously variable transmission to skip from a normal running condition to an abruptly accelerated or decelerated condition for abruptly accelerating or decelerating the motor vehicle is referred to as a "skip control process".
The applicant has also proposed a method of controlling the speed reduction ratio of a continuously variable transmission as disclosed in Japanese laid-open patent publication No. 2-249727. According to the disclosed control method, a target acceleration to be reached is established depending on the accelerator opening, and a target acceleration is established which is required to change the present acceleration to the target acceleration to be reached with desired characteristics. The continuously variable transmission is then controlled to obtain the target acceleration.
To carry out the above control method, a component (referred to as "acceleration term di(G)") corresponding to the difference between the target acceleration and the present acceleration is established, and the rate of change di/dt of the speed reduction ratio is determined by adding the acceleration term di(G) to the sum of the power term di(P) and the engine rotation term di(Ne).
Specifically, when the driver depresses the accelerator pedal so as to accelerate the motor vehicle, the rate of change di/dt of the speed reduction ratio which is required by the depression of the accelerator pedal is calculated, and the calculated value is applied to an actuator for controlling the speed reduction ratio. The control method is thus capable of controlling the speed reduction ratio in a manner to meet the request from the driver even while the motor vehicle is running in a transient state. However, a certain period of time elapses after the depression of the accelerator pedal is detected until the rate of change di/dt of the speed reduction ratio is calculated and applied to the actuator, and the actuator itself starts to operate with a certain time lag. Because of such time lags, the motor vehicle starts to be accelerated with a time lag in response to the depression of the accelerator pedal, with the result that the response of the motor vehicle to the depression of the accelerator pedal is relatively poor.